1. Field of the Invention
The present invention relates to an objective lens system for endoscopes.
2. Description of the Prior Art
As an objective lens system for endoscopes, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-173415. This objective lens system has defects that the lens system has a long total length, that the outside diameter of the lens system is large as compared with the diameter of an image formed thereby, and so on.
Further, as another conventional example of objective lens systems, there is also known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-26123. This objective lens system has defects that the lens system has a narrow field angle and that the lens system produces remarkable distortion which deforms the marginal portion of an image formed thereby.
Furthermore, used as endoscopes are fiber scopes which use image guides and, in addition, video scopes which use solid-state image pickup devices such as CCD's in the distal ends thereof.
In the recent years where video scopes have smaller outside diameters owing to compacter designs of image pickup devices, there are developed video scopes for observing interiors of bronchial tubes and urinary organs.
Video scopes do not permit reproducing colors or images formed on the monitors therefor since the solid-state image pickup devices have sensitivity not only to visible rays but also to infrared rays. It is therefore necessary to arrange filters for shielding the infrared rays in the optical systems of video scopes. Further, when a laser ranging from near infared to far infrared regions is used for medical treatment through video scopes, the laser saturates the CCD's, thereby hindering observation of locations to be treated due to smear and blooming. It is therefore necessary to arrange filters for cutting off wavelengths of the laser in the objective optical systems.
Thickness of the infrared cut filters, CCD cover glass, laser shielding filters, etc. remains unchanged even after the image pickup devices have compacter designs and the optical systems have shorter focal lengths. Accordingly, the optical path lengths as measured in the direction of the thickness of the filters are long relative to the focal lengths.
As an objective lens system for endoscopes comprising filters in the optical system thereof, there is known the lens system which has the composition illustrated in FIG. 1. This optical system is usually of the retrofocus type and satisfies the following relationship. EQU I=fsin .theta.
wherein the reference symbol I represents the image height, the reference symbol f designates the focal length of the optical system and the reference symbol .theta. denotes the half field angle of the optical system.
In order to arrange filters, F.sub.1, F.sub.2 and a cover glass C which have long optical path lengths as compared with the focal length f in this type of lens system, it is necessary to select a high retro ratio (a value obtained by dividing the distance as measured from the final surface to the image surface of a lens system by the focal length thereof). When a high retro ratio is selected, without prolonging total length, for a lens system which has a short total length, each of the lens components has strengthened power, thereby making it difficult to correct aberrations. Since focal length of the lens system is further shortened especially by widening field angle thereof, it is impossible to widen the field angle of the lens system.
When an attempt is made to correct aberrations favorably in this type of lens system, the lens system comprises a larger number of lens components, a longer total length accordingly and becomes unsuited for use with endoscopes for observing interiors of thin tubes.
The absorption type of infrared cut filters are usually made of phosphate glass materials. Since the phosphate glass materials have low resistance to humidity, these materials must always be coated so as to prevent contact with ambient air. The coated filters may be injured on the circumferences thereof or the coating may peel off and, in such a condition, the injured portion is exposed to ambient air and may allow the filters to become whitely turbid from the marginal portions thereof. It is necessary to design the filters in such a manner that rays are incident on the filters at the portions rather lower than the outside diameters of the filters. However, in the above-described type of lens system wherein the two filters F.sub.1 and F.sub.2 are arranged before the CCD cover glass C, rays are incident on the filters at heights which are nearly the same as the image height or higher than the image height, and since the filters are arranged at locations where rays are highest in the lens system, it is impossible to set the heights of rays at levels sufficiently lower than the diameters of the filters.